Generally the running water sound under the dash is air trapped in the coolant system. This air then circulates through the heater core. Just check the coolant level in the radiator and make sure the overflow reservoir is full. Run a few heating and cooling cycles of the engine and recheck. This should help take care of the sound.
On our 2000 Navigator, it's under the driver's side dash under the steering wheel.
under the air filter
passenger side under the dash
Under the window washer tank
It is located UNDER the windshield washer reservoir
Have your heater core check out. On my navigator there was a leakage in a tube under the passenger fender. Take it off and check out you tubes.
Under the front seats
On a 2003 Lincoln Navigator : The check engine light / service engine soon light is a drawing of an engine located directly under the center of the tachometer
It is on the back of the engine under the wiper area
The diagnostic connector is under the dash on the driver side.
On a 2004 Lincoln Navigator : ( NO , the passenger compartment fuse panel and the power distribution box are combined behind the kick panel in the front passenger footwell )
Check for spark plug blown out. Cylinder head repair may be needed as well as replacement of coil and fuel rail. This will cost a few dollars.